1. Field of the Invention
This invention relates generally to the field of rigid disc drive data storage devices and more particularly, but not by way of limitation, to a limit stop for defining the range of motion of the disc drive actuator.
2. Brief Description of the Prior Art
Disc drive data storage devices have found widespread use as a cost effective and convenient mechanism for the storage of large computer programs and user generated data. Those types of disc drives known as "Winchester" disc drives incorporate rigid media, and the read/write heads "fly" above the disc surface on a thin layer of air dragged along by the spinning of the discs. An actuator mechanism is included to precisely move the read/write heads from one of a large number of circular, concentric tracks to another, in order to access any data recorded on the disc surface.
Market demands for increased storage capacity and decreased access time have lead to the almost exclusive use of voice coil motors (VCMs) to drive the actuator which positions the heads, while recent trends toward smaller and smaller form factors have lead to the preeminence of rotary VCM actuators. In a typical rotary VCM actuator, an array of permanent magnets with flux-concentrating pole pieces is fixedly mounted to a housing base or base plate, with a horizontal air gap provided which is the location of a high concentration of magnetic flux. A horizontal coil mounted to a pivot housing, which turns about a vertical axis, is suspended in the magnetic field, and an array of read/write heads is cantilevered from the side of the pivot housing opposite the coil and over the stack of discs. Controlled DC power is applied to the ends of the coil, and the polarity and magnitude of the DC voltage determines the direction and speed of the movement of the heads in accordance with the well-known Lorentz relationship.
Since VCM actuators have no inherent magnetic detent when power is removed from the disc drive, such disc drives must incorporate some sort of latching mechanism to hold the heads in a stationary relationship to the discs when power is removed, in order to prevent damaging relative movement between the heads and discs while the discs are not spinning. Similarly, some device must also be included to limit the range of motion of the actuator, should a "runaway" condition occur in the electronics controlling the coil power. Since VCM actuators are subject to movement whenever a DC current is applied to the coil, any failure of the electronic circuitry supplying the coil current which causes a constant current to the coil will result in the actuator being driven to one or the other of the extremes of its range of motion, dependent on the polarity of the applied signal.
The design of a limit stop to define the extreme limit of the range of motion for a voice coil actuator involves several considerations: while the mass of the moving portion of the actuator and the maximum velocity achievable by the actuator are known quantities, a serviceable limit stop must be a compromise between total rigidity and a minimal compliance needed to prevent damage to the delicate components of the flexure assembly mounting the heads of the drive. That is, if the limit stop is too rigid, the inertia of the head can cause permanent deformation to the gimbal elements of the flexure if the actuator is brought to an instantaneous total stop. Similarly, if the limit stop is too compliant, undesirable oscillations of the heads over the surface of the disc can occur, presenting another possible mode for component damage.
Many prior art limit stops thus incorporate contact components made from rubbers or synthetic polymers. Care must be exercised in selecting such materials, however, to ensure that out-gassing does not occur, since large molecular chains can be large enough to build up on the discs or air-bearing surfaces of the heads, causing damage to these critical components. Furthermore, such soft "bumper" elements must be mounted on a rigid support member to provide the necessary stiffness to the entire assembly. Clearly such multi-part limit stops entail a number of components and some assembly, as well.
It can be seen that a need exists for a limit stop which is inexpensive to manufacture, simple to install and easily adaptable to various designs of disc drive data storage devices.